FIG. 1 illustrates a digital-analog conversion device of a switched capacitor filter (SCF) type according to a ΔΣ modulation system (hereinafter, referred to as an SCF-type ΔΣ DAC). As illustrated in FIG. 1, an SCF-type ΔΣ DAC 100 includes: a ΔΣ modulator 101 that executes ΔΣ modulation for input digital data; a DWA (data-weighted-averaging) processor 102 that converts a PDM signal output from the ΔΣ modulator 101 into a thermometer code, rotates the thermometer code, and outputs the rotated thermometer code; and an SCF 103 to which digital data output from the DWA processor 102 is input. The SCF-type ΔΣ DAC 100 is a device that converts input digital data into analog data and outputs the converted data.
An audio ΔΣ DAC for high end uses is particularly required to have a high SN ratio. Accordingly, it is necessary to suppress a system noise of the ΔΣ modulator 101, a kT/C noise of the SCF 103, and the like. The ΔΣ modulator 101 can shift a low-frequency noise to a high-frequency noise based on the noise shaping characteristics thereof and thus, can reduce the amount of noise within the frequency band.
The SN ratio of the ΔΣ modulator 101 is determined based on an order L, an oversampling ratio (OSR) M, and the number B of output bits. Equation (1) represents an equation for calculating the SN ratio.
                              [                      Formula            ⁢                                                  ⁢            1                    ]                ⁢                                                                                      SN        =                  10          ⁢                                          ⁢                      log            ⁡                          (                                                3                  2                                ⁢                                                                            2                      ⁢                                                                                          ⁢                      L                                        +                    1                                                        π                                          2                      ⁢                                                                                          ⁢                      L                                                                      ⁢                                                                            M                                                                        2                          ⁢                                                                                                          ⁢                          L                                                +                        1                                                              ⁡                                          (                                                                        2                          B                                                -                        1                                            )                                                        2                                            )                                                          Equation        ⁢                                  ⁢                  (          1          )                    
For the achievement of high performance of the ΔΣ modulator 101, particularly the SN ratio can be improved considerably by increasing the OSR. In Patent literature 1, a ΔΣ DAC using a high OSR is disclosed. FIG. 2 illustrates the noise shaping characteristic of a ΔΣ modulator of the third order and the noise shaping characteristic at the time of doubling the OSR. As illustrated in FIG. 2, by doubling the OSR, the amount of noise within the frequency band (within fb=20 kHz) can be suppressed.
In addition, for example, in Patent literature 2, a ΔΣ modulator including a switched capacitor circuit is disclosed. Furthermore, in Patent literature 3, a ΔΣ-type D/A converter including a ΔΣ modulator is disclosed.